You don't have to make a manual exposure all the time but it is a huge advantage to know how to do it if you're in a situation where your camera gets confused by very bright or dark subjects and sets the wrong exposure (http://www.photographycourses.biz/wro...) which will happen sometimes.

A manual exposure is made by balancing the size of your aperture, the length od time your shutter is open and how sensitive your camera is with your ISO. Watch the tutorial and I'll explain.